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perlsec ()
  • >> perlsec (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perlsec (1) ( Разные man: Команды и прикладные программы пользовательского уровня )
  • 
    
    

    NAME

         perlsec - Perl security
    
    
    

    DESCRIPTION

         Perl is designed to make it easy to program securely even
         when running with extra privileges, like setuid or setgid
         programs.  Unlike most command line shells, which are based
         on multiple substitution passes on each line of the script,
         Perl uses a more conventional evaluation scheme with fewer
         hidden snags.  Additionally, because the language has more
         builtin functionality, it can rely less upon external (and
         possibly untrustworthy) programs to accomplish its purposes.
    
         Perl automatically enables a set of special security checks,
         called taint mode, when it detects its program running with
         differing real and effective user or group IDs.  The setuid
         bit in Unix permissions is mode 04000, the setgid bit mode
         02000; either or both may be set.  You can also enable taint
         mode explicitly by using the -T command line flag. This flag
         is strongly suggested for server programs and any program
         run on behalf of someone else, such as a CGI script. Once
         taint mode is on, it's on for the remainder of your script.
    
         While in this mode, Perl takes special precautions called
         taint checks to prevent both obvious and subtle traps.  Some
         of these checks are reasonably simple, such as verifying
         that path directories aren't writable by others; careful
         programmers have always used checks like these.  Other
         checks, however, are best supported by the language itself,
         and it is these checks especially that contribute to making
         a set-id Perl program more secure than the corresponding C
         program.
    
         You may not use data derived from outside your program to
         affect something else outside your program--at least, not by
         accident.  All command line arguments, environment
         variables, locale information (see the perllocale manpage),
         results of certain system calls (readdir(), readlink(), the
         variable of shmread(), the messages returned by msgrcv(),
         the password, gcos and shell fields returned by the
         getpwxxx() calls), and all file input are marked as
         "tainted".  Tainted data may not be used directly or
         indirectly in any command that invokes a sub-shell, nor in
         any command that modifies files, directories, or processes.
         (Important exception: If you pass a list of arguments to
         either `system' or `exec', the elements of that list are NOT
         checked for taintedness.) Any variable set to a value
         derived from tainted data will itself be tainted, even if it
         is logically impossible for the tainted data to alter the
         variable.  Because taintedness is associated with each
         scalar value, some elements of an array can be tainted and
         others not.
         For example:
    
             $arg = shift;               # $arg is tainted
             $hid = $arg, 'bar';         # $hid is also tainted
             $line = <>;                 # Tainted
             $line = <STDIN>;            # Also tainted
             open FOO, "/home/me/bar" or die $!;
             $line = <FOO>;              # Still tainted
             $path = $ENV{'PATH'};       # Tainted, but see below
             $data = 'abc';              # Not tainted
    
             system "echo $arg";         # Insecure
             system "/bin/echo", $arg;   # Secure (doesn't use sh)
             system "echo $hid";         # Insecure
             system "echo $data";        # Insecure until PATH set
    
             $path = $ENV{'PATH'};       # $path now tainted
    
             $ENV{'PATH'} = '/bin:/usr/bin';
             delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};
    
             $path = $ENV{'PATH'};       # $path now NOT tainted
             system "echo $data";        # Is secure now!
    
             open(FOO, "< $arg");        # OK - read-only file
             open(FOO, "> $arg");        # Not OK - trying to write
    
             open(FOO,"echo $arg|");     # Not OK, but...
             open(FOO,"-|")
                 or exec 'echo', $arg;   # OK
    
             $shout = `echo $arg`;       # Insecure, $shout now tainted
    
             unlink $data, $arg;         # Insecure
             umask $arg;                 # Insecure
    
             exec "echo $arg";           # Insecure
             exec "echo", $arg;          # Secure (doesn't use the shell)
             exec "sh", '-c', $arg;      # Considered secure, alas!
    
             @files = <*.c>;             # insecure (uses readdir() or similar)
             @files = glob('*.c');       # insecure (uses readdir() or similar)
    
         If you try to do something insecure, you will get a fatal
         error saying something like "Insecure dependency" or
         "Insecure $ENV{PATH}".  Note that you can still write an
         insecure system or exec, but only by explicitly doing
         something like the "considered secure" example above.
    
    
    
         Laundering and Detecting Tainted Data
    
         To test whether a variable contains tainted data, and whose
         use would thus trigger an "Insecure dependency" message,
         check your nearby CPAN mirror for the Taint.pm module, which
         should become available around November 1997.  Or you may be
         able to use the following is_tainted() function.
    
             sub is_tainted {
                 return ! eval {
                     join('',@_), kill 0;
                     1;
                 };
             }
    
         This function makes use of the fact that the presence of
         tainted data anywhere within an expression renders the
         entire expression tainted.  It would be inefficient for
         every operator to test every argument for taintedness.
         Instead, the slightly more efficient and conservative
         approach is used that if any tainted value has been accessed
         within the same expression, the whole expression is
         considered tainted.
    
         But testing for taintedness gets you only so far.  Sometimes
         you have just to clear your data's taintedness.  The only
         way to bypass the tainting mechanism is by referencing
         subpatterns from a regular expression match.  Perl presumes
         that if you reference a substring using $1, $2, etc., that
         you knew what you were doing when you wrote the pattern.
         That means using a bit of thought--don't just blindly
         untaint anything, or you defeat the entire mechanism.  It's
         better to verify that the variable has only good characters
         (for certain values of "good") rather than checking whether
         it has any bad characters.  That's because it's far too easy
         to miss bad characters that you never thought of.
    
         Here's a test to make sure that the data contains nothing
         but "word" characters (alphabetics, numerics, and
         underscores), a hyphen, an at sign, or a dot.
    
             if ($data =~ /^([-\@\w.]+)$/) {
                 $data = $1;                     # $data now untainted
             } else {
                 die "Bad data in $data";        # log this somewhere
             }
    
         This is fairly secure because `/\w+/' doesn't normally match
         shell metacharacters, nor are dot, dash, or at going to mean
         something special to the shell.  Use of `/.+/' would have
         been insecure in theory because it lets everything through,
         but Perl doesn't check for that.  The lesson is that when
         untainting, you must be exceedingly careful with your
         patterns.  Laundering data using regular expression is the
         only mechanism for untainting dirty data, unless you use the
         strategy detailed below to fork a child of lesser privilege.
    
         The example does not untaint $data if `use locale' is in
         effect, because the characters matched by `\w' are
         determined by the locale.  Perl considers that locale
         definitions are untrustworthy because they contain data from
         outside the program.  If you are writing a locale-aware
         program, and want to launder data with a regular expression
         containing `\w', put `no locale' ahead of the expression in
         the same block.  See the SECURITY entry in the perllocale
         manpage for further discussion and examples.
    
         Switches On the "#!" Line
    
         When you make a script executable, in order to make it
         usable as a command, the system will pass switches to perl
         from the script's #!  line.  Perl checks that any command
         line switches given to a setuid (or setgid) script actually
         match the ones set on the #! line.  Some Unix and Unix-like
         environments impose a one-switch limit on the #!  line, so
         you may need to use something like `-wU' instead of `-w -U'
         under such systems.  (This issue should arise only in Unix
         or Unix-like environments that support #! and setuid or
         setgid scripts.)
    
         Cleaning Up Your Path
    
         For "Insecure `$ENV{PATH}'" messages, you need to set
         `$ENV{'PATH'}' to a known value, and each directory in the
         path must be non-writable by others than its owner and
         group.  You may be surprised to get this message even if the
         pathname to your executable is fully qualified.  This is not
         generated because you didn't supply a full path to the
         program; instead, it's generated because you never set your
         PATH environment variable, or you didn't set it to something
         that was safe.  Because Perl can't guarantee that the
         executable in question isn't itself going to turn around and
         execute some other program that is dependent on your PATH,
         it makes sure you set the PATH.
    
         The PATH isn't the only environment variable which can cause
         problems.  Because some shells may use the variables IFS,
         CDPATH, ENV, and BASH_ENV, Perl checks that those are either
         empty or untainted when starting subprocesses. You may wish
         to add something like this to your setid and taint-checking
         scripts.
    
             delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};   # Make %ENV safer
    
         It's also possible to get into trouble with other operations
         that don't care whether they use tainted values.  Make
         judicious use of the file tests in dealing with any user-
         supplied filenames.  When possible, do opens and such after
         properly dropping any special user (or group!)  privileges.
         Perl doesn't prevent you from opening tainted filenames for
         reading, so be careful what you print out.  The tainting
         mechanism is intended to prevent stupid mistakes, not to
         remove the need for thought.
    
         Perl does not call the shell to expand wild cards when you
         pass system and exec explicit parameter lists instead of
         strings with possible shell wildcards in them.
         Unfortunately, the open, glob, and backtick functions
         provide no such alternate calling convention, so more
         subterfuge will be required.
    
         Perl provides a reasonably safe way to open a file or pipe
         from a setuid or setgid program: just create a child process
         with reduced privilege who does the dirty work for you.
         First, fork a child using the special open syntax that
         connects the parent and child by a pipe.  Now the child
         resets its ID set and any other per-process attributes, like
         environment variables, umasks, current working directories,
         back to the originals or known safe values.  Then the child
         process, which no longer has any special permissions, does
         the open or other system call.  Finally, the child passes
         the data it managed to access back to the parent.  Because
         the file or pipe was opened in the child while running under
         less privilege than the parent, it's not apt to be tricked
         into doing something it shouldn't.
    
         Here's a way to do backticks reasonably safely.  Notice how
         the exec is not called with a string that the shell could
         expand.  This is by far the best way to call something that
         might be subjected to shell escapes: just never call the
         shell at all.
    
    
    
             use English;
             die "Can't fork: $!" unless defined $pid = open(KID, "-|");
             if ($pid) {           # parent
                 while (<KID>) {
                     # do something
                 }
                 close KID;
             } else {
                 my @temp = ($EUID, $EGID);
                 $EUID = $UID;
                 $EGID = $GID;    #      initgroups() also called!
                 # Make sure privs are really gone
                 ($EUID, $EGID) = @temp;
                 die "Can't drop privileges"
                         unless $UID == $EUID  && $GID eq $EGID;
                 $ENV{PATH} = "/bin:/usr/bin";
                 exec 'myprog', 'arg1', 'arg2'
                     or die "can't exec myprog: $!";
             }
    
         A similar strategy would work for wildcard expansion via
         `glob', although you can use `readdir' instead.
    
         Taint checking is most useful when although you trust
         yourself not to have written a program to give away the
         farm, you don't necessarily trust those who end up using it
         not to try to trick it into doing something bad.  This is
         the kind of security checking that's useful for set-id
         programs and programs launched on someone else's behalf,
         like CGI programs.
    
         This is quite different, however, from not even trusting the
         writer of the code not to try to do something evil.  That's
         the kind of trust needed when someone hands you a program
         you've never seen before and says, "Here, run this."  For
         that kind of safety, check out the Safe module, included
         standard in the Perl distribution.  This module allows the
         programmer to set up special compartments in which all
         system operations are trapped and namespace access is
         carefully controlled.
    
         Security Bugs
    
         Beyond the obvious problems that stem from giving special
         privileges to systems as flexible as scripts, on many
         versions of Unix, set-id scripts are inherently insecure
         right from the start.  The problem is a race condition in
         the kernel.  Between the time the kernel opens the file to
         see which interpreter to run and when the (now-set-id)
         interpreter turns around and reopens the file to interpret
         it, the file in question may have changed, especially if you
         have symbolic links on your system.
         Fortunately, sometimes this kernel "feature" can be
         disabled.  Unfortunately, there are two ways to disable it.
         The system can simply outlaw scripts with any set-id bit
         set, which doesn't help much.  Alternately, it can simply
         ignore the set-id bits on scripts.  If the latter is true,
         Perl can emulate the setuid and setgid mechanism when it
         notices the otherwise useless setuid/gid bits on Perl
         scripts.  It does this via a special executable called
         suidperl that is automatically invoked for you if it's
         needed.
    
         However, if the kernel set-id script feature isn't disabled,
         Perl will complain loudly that your set-id script is
         insecure.  You'll need to either disable the kernel set-id
         script feature, or put a C wrapper around the script.  A C
         wrapper is just a compiled program that does nothing except
         call your Perl program.   Compiled programs are not subject
         to the kernel bug that plagues set-id scripts.  Here's a
         simple wrapper, written in C:
    
             #define REAL_PATH "/path/to/script"
             main(ac, av)
                 char **av;
             {
                 execv(REAL_PATH, av);
             }
    
         Compile this wrapper into a binary executable and then make
         it rather than your script setuid or setgid.
    
         See the program wrapsuid in the eg directory of your Perl
         distribution for a convenient way to do this automatically
         for all your setuid Perl programs.  It moves setuid scripts
         into files with the same name plus a leading dot, and then
         compiles a wrapper like the one above for each of them.
    
         In recent years, vendors have begun to supply systems free
         of this inherent security bug.  On such systems, when the
         kernel passes the name of the set-id script to open to the
         interpreter, rather than using a pathname subject to
         meddling, it instead passes /dev/fd/3.  This is a special
         file already opened on the script, so that there can be no
         race condition for evil scripts to exploit.  On these
         systems, Perl should be compiled with
         `-DSETUID_SCRIPTS_ARE_SECURE_NOW'.  The Configure program
         that builds Perl tries to figure this out for itself, so you
         should never have to specify this yourself.  Most modern
         releases of SysVr4 and BSD 4.4 use this approach to avoid
         the kernel race condition.
    
         Prior to release 5.003 of Perl, a bug in the code of
         suidperl could introduce a security hole in systems compiled
         with strict POSIX compliance.
    
         Protecting Your Programs
    
         There are a number of ways to hide the source to your Perl
         programs, with varying levels of "security".
    
         First of all, however, you can't take away read permission,
         because the source code has to be readable in order to be
         compiled and interpreted.  (That doesn't mean that a CGI
         script's source is readable by people on the web, though.)
         So you have to leave the permissions at the socially
         friendly 0755 level.  This lets people on your local system
         only see your source.
    
         Some people mistakenly regard this as a security problem.
         If your program does insecure things, and relies on people
         not knowing how to exploit those insecurities, it is not
         secure.  It is often possible for someone to determine the
         insecure things and exploit them without viewing the source.
         Security through obscurity, the name for hiding your bugs
         instead of fixing them, is little security indeed.
    
         You can try using encryption via source filters (Filter::*
         from CPAN).  But crackers might be able to decrypt it.  You
         can try using the byte code compiler and interpreter
         described below, but crackers might be able to de-compile
         it.  You can try using the native-code compiler described
         below, but crackers might be able to disassemble it.  These
         pose varying degrees of difficulty to people wanting to get
         at your code, but none can definitively conceal it (this is
         true of every language, not just Perl).
    
         If you're concerned about people profiting from your code,
         then the bottom line is that nothing but a restrictive
         licence will give you legal security.  License your software
         and pepper it with threatening statements like "This is
         unpublished proprietary software of XYZ Corp.  Your access
         to it does not give you permission to use it blah blah
         blah."  You should see a lawyer to be sure your licence's
         wording will stand up in court.
    
    
    

    SEE ALSO

         the perlrun manpage for its description of cleaning up
         environment variables.
    
    
    
    


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